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[91.219.240.2]) by smtp.gmail.com with ESMTPSA id ew13-20020a056402538d00b004642b35f89esm5950875edb.9.2022.11.15.01.30.14 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 15 Nov 2022 01:30:16 -0800 (PST) From: Vitaly Kuznetsov To: Sean Christopherson Cc: James Morse , Alexandru Elisei , Suzuki K Poulose , Oliver Upton , Atish Patra , David Hildenbrand , kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, kvmarm@lists.cs.columbia.edu, linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, Isaku Yamahata , Fabiano Rosas , Michael Ellerman , Chao Gao , Thomas Gleixner , Yuan Yao , Paolo Bonzini , Marc Zyngier , Huacai Chen , Aleksandar Markovic , Anup Patel , Paul Walmsley , Palmer Dabbelt , Albert Ou , Christian Borntraeger , Janosch Frank , Claudio Imbrenda , Matthew Rosato , Eric Farman Subject: Re: [PATCH 10/44] KVM: VMX: Clean up eVMCS enabling if KVM initialization fails In-Reply-To: References: <20221102231911.3107438-1-seanjc@google.com> <20221102231911.3107438-11-seanjc@google.com> <87mt98qfi2.fsf@ovpn-194-252.brq.redhat.com> Date: Tue, 15 Nov 2022 10:30:14 +0100 Message-ID: <87sfikmuop.fsf@redhat.com> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-mips@vger.kernel.org Sean Christopherson writes: > On Thu, Nov 03, 2022, Vitaly Kuznetsov wrote: >> Sean Christopherson writes: >> > + /* >> > + * Reset everything to support using non-enlightened VMCS access later >> > + * (e.g. when we reload the module with enlightened_vmcs=0) >> > + */ >> > + for_each_online_cpu(cpu) { >> > + vp_ap = hv_get_vp_assist_page(cpu); >> > + >> > + if (!vp_ap) >> > + continue; >> > + >> > + vp_ap->nested_control.features.directhypercall = 0; >> > + vp_ap->current_nested_vmcs = 0; >> > + vp_ap->enlighten_vmentry = 0; >> > + } >> >> Unrelated to your patch but while looking at this code I got curious >> about why don't we need a protection against CPU offlining here. Turns >> out that even when we offline a CPU, its VP assist page remains >> allocated (see hv_cpu_die()), we just write '0' to the MSR and thus > > Heh, "die". Hyper-V is quite dramatic. > >> accessing the page is safe. The consequent hv_cpu_init(), however, does >> not restore VP assist page when it's already allocated: >> >> # rdmsr -p 24 0x40000073 >> 10212f001 >> # echo 0 > /sys/devices/system/cpu/cpu24/online >> # echo 1 > /sys/devices/system/cpu/cpu24/online >> # rdmsr -p 24 0x40000073 >> 0 >> >> The culprit is commit e5d9b714fe402 ("x86/hyperv: fix root partition >> faults when writing to VP assist page MSR"). A patch is inbound. >> >> 'hv_root_partition' case is different though. We do memunmap() and reset >> VP assist page to zero so it is theoretically possible we're going to >> clash. Unless I'm missing some obvious reason why module unload can't >> coincide with CPU offlining, we may be better off surrounding this with >> cpus_read_lock()/cpus_read_unlock(). > > I finally see what you're concerned about. If a CPU goes offline and its assist > page is unmapped, zeroing out the nested/eVMCS stuff will fault. > > I think the real problem is that the purging of the eVMCS is in the wrong place. > Move the clearing to vmx_hardware_disable() and then the CPU hotplug bug goes > away once KVM disables hotplug during hardware enabling/disable later in the series. > There's no need to wait until module exit, e.g. it's not like it costs much to > clear a few variables, and IIUC the state is used only when KVM is actively using > VMX/eVMCS. > > However, I believe there's a second bug. KVM's CPU online hook is called before > Hyper-V's online hook (CPUHP_AP_ONLINE_DYN). Before this series, which moves KVM's > hook from STARTING to ONLINE, KVM's hook is waaaay before Hyper-V's. That means > that hv_cpu_init()'s allocation of the VP assist page will come _after_ KVM's > check in vmx_hardware_enable() > > /* > * This can happen if we hot-added a CPU but failed to allocate > * VP assist page for it. > */ > if (static_branch_unlikely(&enable_evmcs) && > !hv_get_vp_assist_page(cpu)) > return -EFAULT; > > I.e. CPU hotplug will never work if KVM is running VMs as a Hyper-V guest. I bet > you can repro by doing a SUSPEND+RESUME. > > Can you try to see if that's actually a bug? If so, the only sane fix seems to > be to add a dedicated ONLINE action for Hyper-V. It seems we can't get away without a dedicated stage for Hyper-V anyway, e.g. see our discussion with Michael: https://lore.kernel.org/linux-hyperv/878rkqr7ku.fsf@ovpn-192-136.brq.redhat.com/ All these issues are more or less "theoretical" as there's no real CPU hotplug on Hyper-V/Azure. Yes, it is possible to trigger problems by doing CPU offline/online but I don't see how this may come handy outside of testing envs. > Per patch > > KVM: Rename and move CPUHP_AP_KVM_STARTING to ONLINE section > > from this series, CPUHP_AP_KVM_ONLINE needs to be before CPUHP_AP_SCHED_WAIT_EMPTY > to ensure there are no tasks, i.e. no vCPUs, running on the to-be-unplugged CPU. > > Back to the original bug, proposed fix is below. The other advantage of moving > the reset to hardware disabling is that the "cleanup" is just disabling the static > key, and at that point can simply be deleted as there's no need to disable the > static key when kvm-intel is unloaded since kvm-intel owns the key. I.e. this > patch (that we're replying to) would get replaced with a patch to delete the > disabling of the static key. > >From a quick glance looks good to me, I'll try to find some time to work on this issue. I will likely end up proposing a dedicated CPU hotplug stage for Hyper-V (which needs to happen before KVM's CPUHP_AP_KVM_ONLINE on CPU hotplug and after on unplug) anyway. Thanks for looking into this! > -- > From: Sean Christopherson > Date: Thu, 10 Nov 2022 17:28:08 -0800 > Subject: [PATCH] KVM: VMX: Reset eVMCS controls in VP assist page during > hardware disabling > > Reset the eVMCS controls in the per-CPU VP assist page during hardware > disabling instead of waiting until kvm-intel's module exit. The controls > are activated if and only if KVM creates a VM, i.e. don't need to be > reset if hardware is never enabled. > > Doing the reset during hardware disabling will naturally fix a potential > NULL pointer deref bug once KVM disables CPU hotplug while enabling and > disabling hardware (which is necessary to fix a variety of bugs). If the > kernel is running as the root partition, the VP assist page is unmapped > during CPU hot unplug, and so KVM's clearing of the eVMCS controls needs > to occur with CPU hot(un)plug disabled, otherwise KVM could attempt to > write to a CPU's VP assist page after it's unmapped. > > Reported-by: Vitaly Kuznetsov > Signed-off-by: Sean Christopherson > --- > arch/x86/kvm/vmx/vmx.c | 50 +++++++++++++++++++++++++----------------- > 1 file changed, 30 insertions(+), 20 deletions(-) > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c > index aca88524fd1e..ae13aa3e8a1d 100644 > --- a/arch/x86/kvm/vmx/vmx.c > +++ b/arch/x86/kvm/vmx/vmx.c > @@ -552,6 +552,33 @@ static int hv_enable_direct_tlbflush(struct kvm_vcpu *vcpu) > return 0; > } > > +static void hv_reset_evmcs(void) > +{ > + struct hv_vp_assist_page *vp_ap; > + > + if (!static_branch_unlikely(&enable_evmcs)) > + return; > + > + /* > + * KVM should enable eVMCS if and only if all CPUs have a VP assist > + * page, and should reject CPU onlining if eVMCS is enabled the CPU > + * doesn't have a VP assist page allocated. > + */ > + vp_ap = hv_get_vp_assist_page(smp_processor_id()); > + if (WARN_ON_ONCE(!vp_ap)) > + return; > + > + /* > + * Reset everything to support using non-enlightened VMCS access later > + * (e.g. when we reload the module with enlightened_vmcs=0) > + */ > + vp_ap->nested_control.features.directhypercall = 0; > + vp_ap->current_nested_vmcs = 0; > + vp_ap->enlighten_vmentry = 0; > +} > + > +#else /* IS_ENABLED(CONFIG_HYPERV) */ > +static void hv_reset_evmcs(void) {} > #endif /* IS_ENABLED(CONFIG_HYPERV) */ > > /* > @@ -2497,6 +2524,8 @@ static void vmx_hardware_disable(void) > if (cpu_vmxoff()) > kvm_spurious_fault(); > > + hv_reset_evmcs(); > + > intel_pt_handle_vmx(0); > } > > @@ -8463,27 +8492,8 @@ static void vmx_exit(void) > kvm_exit(); > > #if IS_ENABLED(CONFIG_HYPERV) > - if (static_branch_unlikely(&enable_evmcs)) { > - int cpu; > - struct hv_vp_assist_page *vp_ap; > - /* > - * Reset everything to support using non-enlightened VMCS > - * access later (e.g. when we reload the module with > - * enlightened_vmcs=0) > - */ > - for_each_online_cpu(cpu) { > - vp_ap = hv_get_vp_assist_page(cpu); > - > - if (!vp_ap) > - continue; > - > - vp_ap->nested_control.features.directhypercall = 0; > - vp_ap->current_nested_vmcs = 0; > - vp_ap->enlighten_vmentry = 0; > - } > - > + if (static_branch_unlikely(&enable_evmcs)) > static_branch_disable(&enable_evmcs); > - } > #endif > vmx_cleanup_l1d_flush(); > > > base-commit: 5f47ba6894477dfbdc5416467a25fb7acb47d404 -- Vitaly